There has been significant interest in altering fatty acid (FA) metabolism in plants in order to create plant-derived oils designed for specific purposes. The properties of the oil are determined by its fatty acid composition, which affects both nutritional composition and oxidative stability.
The level of saturated FAs in various types of fats and oils is a major health concern. Hence, there has been increasing pressure in the market to provide plant oils with lower saturated FA content. The main components of saturated fatty acid in most vegetable oil are 16:0 (palmitic acid) and 18:0 (stearic acid).
In the vegetable oil market, oil having less than 7% saturated FA content can be labeled “low-sat” and oil having less than 3.5% saturated FA content can be labelled “no-sat”. Canola (Brassica napus) seed oil is typically low in saturated fatty acid, but it is difficult to keep the saturated fatty acid level below the “low-sat” threshold of 7% saturated FA content.
Previous attempts have been made to address this problem. For example, transgenic plants have been made that contain heterologous plant genes involved in fatty acid metabolism (see for example: Shah S, Weselake R (2003) Farming For the Future, AARI project #19990032, Final Report, pp. 1-82; and Yao et al. Plant Biotech J 2003, 1:221). However, these transgenic plants showed little or no reduction of saturated fatty acid in the transgenic plant. For example, Yao et al. (2003) report a 1 to 2% decrease in saturated FA level associated with expression of the ADS1 gene from Arabidopsis in B. juncea seeds.
In this context, prokaryotic genes provide an attractive alternative to plant genes, however prokaryotic proteins often show limited or no activity in a plant background (see e.g. Hahn J J, Eschenlauer A C, Narrol M H, Somers D A, Srienc F (1997) Growth kinetics, nutrient uptake, and expression of the Alcaligenes eutrophus poly(β-hydroxybutyrate) synthesis pathway in transgenic maize cell suspension cultures. Biotech Prog 13: 347-354).
It has been shown previously that the nutritional value of plant seed oil can be improved by making transgenic plants that express a heterologous delta-6 desaturase enzyme (derived from cyanobacteria, borage, or evening primrose) to 5 effect the conversion of linoleic acid (ClsA9, 12), a polyunsaturated fatty acid, to gamma-linolenic acid (GLA, Cl8A6,9,12) (see U.S. Pat. Nos. 5,552,306; 5,614,393; 5,663,068; 5,689,050 5,789,220; 6,355,861; 6,683,232; and us patent application publication No.: 20040078845). Linoleic acid (ClsA9,12) is an LO essential dietary constituent that cannot be synthesized by vertebrates and is usually obtained from plant sources; vertebrate cells can introduce double bonds at the delta-9 position of fatty acids but cannot introduce additional double bonds between the delta-9 double bond and the methyl-15 terminus of the fatty acid chain. Linoleic acid can be converted by mammals to gamma-linolenic acid (GLA, ClsA6,9,12), which in turn can be converted to arachidonic acid (20:4), an essential precursor of most prostaglandins.
Accordingly, there remains a need for transgenic plants that can provide seed oil having lower levels of saturated fatty acids.